It is a memory safety bug - by using this "indexes as pointers" methodology it is possible to write code where two different owners simultaneously believe they are the sole owner of an object.
Writing that using normal pointers is impossible in safe rust (barring a compiler bug, which do exist but are rare).
You don't get two owners that way. You get something slightly less powerful than owners, which still has all its preconditions satisfied: you can check that it is in bounds, and if so you will find an element of the expected type. You cannot corrupt the underlying allocator's data structures, you cannot resize the array when there are outstanding pointers to its elements, and you cannot violate the type system.
Rust's goal was never to exclude all shared mutability. (Otherwise why support things like locks?) Rather, it excludes only the kinds of shared mutability that open the door to undefined behavior. The point of all these sorts of "workarounds" is that, because there is no longer a single unrestricted kind of shared mutability, you now get to pick which version of restricted mutability you want: reference counting vs bounds checking vs ghostcell's higher-rank lifetimes vs whatever else.
> two different owners simultaneously believe they are the sole owner of an object.
Not in the sense of ownership that matters in Rust. The Vec owns the data. A node with an index in it does not own that data. It merely refers to it.
The key here is when you answer the question, "what happens when I screw it up the indices?" And the answer is logic errors or a panic. Neither of those are memory safety issues.
"memory safety bugs" -> "for example, UAFs" -> "I don't mean a literal UAF" -> "use array index after free" -> 'memory safety is not "code that does not result in UB"'
I mean, you can define "memory safety" to be whatever you want it to be, but the definition everyone else uses (including Rust) is absolutely connected with undefined behavior. More than that, the entire context of this thread assumes that definition. Rust certainly does. And if you are going to use a different definition than everyone else, at least have the courtesy to provide it.
If people used your definition, then it would be wrong to, for example, say that "Java is a memory safe programming language." But that is, as far as I know, widely regarded to be a true statement.
This sort of disagreement is profoundly irritating, because I made it exceptionally clear what I meant from the get-go. All you had to do was respond and say, "oh, it sounds like we are just using different definitions of the term 'memory safety.' if we use your definition, I agree with what you said."
Writing that using normal pointers is impossible in safe rust (barring a compiler bug, which do exist but are rare).